This invention relates to a driving device for a stepping motor, and more particularly to a driving device for a stepping motor which detects an electromotive force (hereinafter referred to as EMF) appearing upon driving of a stepping motor to effect closed-loop control of the stepping motor.
A driving device is already known wherein an EMF induced in a field winding of a stepping motor is detected to detect a position of a rotor in order to effect closed-loop control of the stepping motor in accordance therewith.
Conventionally, stepping motor driving devices of the type mentioned are disclosed, for example, in U.S. Pat. No. 4,282,471 (issued on Aug. 4, 1981) and Japanese laid-open patent No. 59-181999 (laid open on Oct. 16, 1984).
The stepping motor driving device disclosed in U.S. Pat. No. 4,282,471 detects a back EMF induced in a field winding of a non-excited phase by a mutual inductance with a field winding of an excited phase to detect a position of the rotor of a stepping motor.
Meanwhile, the stepping motor driving device disclosed in Japanese laid-open patent No. 59-181999 directly detects, by means of specifically provided sensing means, an EMF induced by a permanent magnet of a stepping motor to detect a position of the rotor of the stepping motor.
On the other hand, the present applicant has already proposed a driving device for a stepping motor which, taking into consideration that an electric current flowing through a field winding of an excited phase of a stepping motor contains a back EMF induced by a self inductance of the winding, detects an EMF induced in the field winding by a magnetic pole of a field magnet of the rotor to detect a position of the rotor of the stepping motor to control rotation of the stepping motor (refer to Japanese Patent Application No. 60-162810).
However, the stepping motor driving device described just above has a drawback that it is necessary to adjust the level and the time constant of a CR circuit because a saturation curve which is to be subtracted from a voltage corresponding to an electric current flowing through a field winding of an excited phase in order to detect a back EMF is produced from the CR circuit.
It has another problem that the circuit construction is complicated and adjustment of the gain and offset of an operational amplifier is necessary for each phase.